Using Rasch measurement theory to explore the fitness for purpose of the genetic counseling outcome scale: a tale of two scales

Background The genetic counseling outcome scale (GCOS-24) is commonly used in clinical genetics to measure patient empowerment; however, there is inconclusive psychometric evidence about this scale. Methods Using data from an urban Canadian clinic where the GCOS-24 is routinely administered before (T1) and 1 month after (T2) genetic counseling, we used Rasch measurement theory (RMT) to test the ordering of response option thresholds, fit, spread of item locations, residual correlations, person separation index (PSI), and stability across time. Results Data from 379 participants showed that the original GCOS-24 items had poor fit to the Rasch model ( χ 2 = 367.8, p